Hehe, that's a good point. There's an old story behind it. A pair of lovers were torn apart, but were granted one day out of the year where they could see each other again. So traditionally, you would ask for a day of good weather since they have to wait another year if the weather is ill...

Oh but that's not really what you asked..! You simply write down your wish. There are bundles of bamboo set up around campus that you can hang them on and decorate.